Fluid Characteristics

 Fluid Characteristics

We discussed that there are different types of fluids, we classify them based on different  characteristics of fluids. In the article, we will discuss the different properties of fluids.

Some of these properties are

• Density
• Viscosity
• Specific Gravity
• Surface Tension
• Vapor Pressure
• Capillarity
• Cohesion
• Adhesion
• Compressibility

Now let take a quick review of all these properties:

1. Density:

The density of the body is defined as the mass per unit volume of liquid. Or it is the ratio between the mass (m)  and volume (V)

Numerically, we can write it as

Its SI units are kg/m³.

2. Viscosity:

Viscosity is an important factor to describe the nature of fluid. It can be defined as the amount of resistance offered by the layers of fluid when shear stress is applied to it.

There are four types of viscosities:

• Dynamic Viscosity.
• Kinematic Viscosity.
• Relative Viscosity.
• Apparent Viscosity.

Before discussing them, I want to clear you the difference between the following terms:

Dynamic and kinematic, Dynamics is the branch of physics which deals with the study of motion and discussing that how the body moves while on the other hand kinematics is the branch of physics deal with the study of motion without discussing the cause of motion.

>Dynamic Viscosity:

Dynamic or absolute viscosity can be stated as the force required to overcome the internal friction in a fluid. Or the force required to move a fluid with a velocity of 1 m/s.

SI unit of dynamic viscosity is 

>Kinematic Viscosity:

Kinematic Viscosity is the ratio of dynamic viscosity to the density of the fluid. It tells us the effect of the gravitational force on a substance’s flow.

SI unit of kinematic viscosity is  

>Relative Viscosity:

The viscosity of a fluid with respect to another fluid is known as relative viscosity.

It is a dimensionless quantity.

>Apparent Viscosity:

For non-Newtonian fluids the viscosity of fluid changes with the change in shear stress. So, we need to specify a single point from whom reference we are measuring the change in the viscosity. The deviation of viscosity from that point is known as apparent viscosity.

3. Specific Gravity:

Specific gravity is the density of a fluid with respect to a reference fluid.

For liquids the reference fluid is water.

For gases the reference fluid is air.

It is also a Dimensionless quantity.

4. Surface Tension:

Surface tension is also an interesting property of the fluid. A fluid consists of different layers of molecules. The free layer or the layer at the interface or the top layer does not have its neighbor molecules. (In the case of top layer molecules above it are absent). This free surface of a liquid behaves like a stretched elastic membrane under tension. This gives rise to the phenomenon of Surface Tension.

5. Vapor Pressure:

In liquid, there is the distribution of energy among molecules. This energy distribution is not equal some molecules have more energy while some has less. The molecules with more energy will break the inter molecular force and will leave the liquid by forming vapors. In a close container, after some time these vapors attain equilibrium. At that time the pressure exerted by the vapors on the surface of the container is known as vapor pressure.

6. Capillarity:

The tendency of a liquid in a capillary tube to rise or fall due to its surface tension is referred to capillarity.

7. Cohesion:

The force of attraction between two molecules of the same substance is called cohesive force.

8. Adhesion:

The force of attraction between two molecules of the different substance is called adhesive force.

9. Compressibility:

The ability of a material to reduce its size or more specifically its volume by the application of pressure on it is termed as compressibility.